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I always prefer to think about the use-case I want to encourage, or that is forced by the nature of the site/audience.
If most of your audience will read the content and immediately want to comment, put it on top.
If on the other hand you want to encourage people to actually read what others have commented before them and only the respond, it makes more ...

My friend at Malmö University, André Mabande, wrote his Bachelor degree on the topic with the title Designing for Dialogue. He concludes that:
The findings in this study seem to validate the hypothesis of the chronology as a
major factor for generating a consistent discussion within a commenting field. When
comments are shown in the order posted ...

To me, this is all about alternatives, providing the "value menu" of things most people would want to say in a tiny comment, in one click.
Provided you have made the alternatives, e.g. "click the +1/awesome/like button!" discoverable and easy, I favor a blocking message like:
We prefer that comments be longer than 15 characters so they add ...

If you expect comments to be part of a conversation, then you should order them from oldest to newest (bottom posting). This follows reading direction, and is far easier to follow a series of related comments. Examples include this site's comments and Reddit.
If you want to emphasise novelty over conversation, you should order comments from newest to ...

I prefer comments to be in chronological order and threaded.
That way, when comments turn into conversations - and they often do, those are easy to follow. Any other order of comments makes it impossible, or at least very hard.
Newest on top means you have to go down to see what is being responded to. And if you decide to start at the bottom, you are ...

In this case, people are commenting about the restaurant rather than responding to comments about the restaurant. So there is no requirement for someone to read the existing comments before adding their own.
In fact people would probably find it quite tedious to trawl through all the comments and would thus be less likely to add their own if the comments ...

It is very poor usability. The box looks like a textarea, and people know how textareas work and are supposed to work. I have seen a significant number of complaints on Facebook because incomplete thoughts are posted and that people do not know how to create paragraphs anymore. Shift+Enter is not intuitive in any way.
I've seen similar complaints on SE as ...

I would recommend a No Limit for the text field upon composing but a suitable limit upon listing.
What I mean is that if the user is limited to expressing their opinion in a regulated fashion the true nature of their opinion could be altered. They may need 752 characters to express their opinion but you've set the limit to 500 characters, what should they ...

It's not so hard if you treat the "infinite nested comments" as another dimension in your design. Most comment threads just have two dimensions: the comment (X = 1) and the number of comments (Y = N). But now you have nested comments, adding Z = M to the mix.
From a UI design point of view this shouldn't represent much of a challenge as there are plenty of ...

You're right, indenting does fail after a certain amount of levels, but you can always go for a solution as what deviantArt does (after about 10 levels, they redirect you to a different page with the whole reply stream).
I'd rather go for changes in font size or grouping panels, though. Here's an image on what I'm talking about. It's simpler and I think it ...

Having a textarea submit on enter breaks with the default behavior. In theory, if you want users to be able to submit a comment by pressing Enter, you should change the textarea to a single line text field. The use of a textarea implies that one should be able to put line breaks in the input.
However, I have to admit I haven't noticed the change. That ...

Commenting is all about context. A few examples of websites encouraging people to generate content
On a social platform like Facebook, asking "What's on your
mind?" in the new-status box is an interactive, clear and
concise way to hint as to what is to be written.
On a knowledge-sharing, social platform like Quora, indicating
topics of interest to the ...

Reddit case study
Let's look at a typical threaded commenting system, such as Reddit. It has the following problems:
It's hard to tell what's the parent of a comment when the tree is very deep.
If the tree is fully expanded, sometimes the parent comment can be off the screen.
There's no indication of which comments are new.
Now let's look at my ...

I built a comment moderation system for a site with 50k new posts a month where each post could have N comments.
Our approach was to use flag for moderation buttons near each post and on each comment, which executed an ajax call to the moderation backend and notified the user of the action. In the backend, moderators had access to two separate lists ...

For the duration of my response, I shall refer to text input as a 1 line form input and textarea as a multi-line form input.
Hypothesis
The textarea or text input themselves do not intrinsically determine if the message can be submitted with the enter key or not. Rather, it is the elements around the textarea or text input that signify its sending ...

From the standpoint of the person filling out the form, I too recommend not setting any limit on how much information the user is allowed to send.
Unfortunately this isn't actually a realistic goal. What happens when a user submits the form with 100k lines of text? Simply put, you don't want users to abuse the feedback form, as it could be detrimental to ...

Well...is it a problem? I forget where I heard this, but there's a story about how everyone assumes those "no dogs" signs are just put up in stores just because they've always been there, it's convention. But really, those signs get put up because someone brought a dog, and it caused problems. To avoid further problems, they banned dogs. Now, maybe they ...

Have you considered offering a recommended limit? For instance, if a comment is less than say, 15 characters, upon submitting the comment the user is presented with an alert suggesting that they elaborate on their comment. This will make the user consider the value of their comment, and will hopefully trigger the action of adding more substance.
Always give ...

It is not bad UX, it is good UX exactly for the reasons you listed.
The site has some business reason to not want to have long comments. It doesn't matter what their reason is. They don't want them, and they know that they can't make the system so watertight so the users won't ever do it.
So what they do is a very good UX. They 1) make it clear to the ...

The UE in me has to ask: What kind of conversation requires infinitely nested comments?
Do they really need to be infinitely nested? Consider Gmail's conversation mode -- it simply groups all replies to the conversation in chronological order. Seems to work pretty well -- it's not changed for years. The only complaint people have is some want an ...

IMO there are activity streams/status casting like Facebook/Twitter's "What's on your mind"/"What’s happening?" where the entries are usually ordered in reverse chronological order to show the most recent items first.
The input for this type of entries is put on top. The user enters the text and the entry is places right under it.
Comments on the other ...

I skip them all the time. I think it’s a kind of system thinking, like in databases, where every table should have id, there somebody think that every thing should have a title, which is not the case for comments, obviously.
And of course, required fields are almost always evil. There’s no way to force users to write a meaningful title, so they will try to ...

I think the usability really needs to be measured against expectation and reversibility.
Obviously it is non-standard behaviour in browsers, so it breaks expectations that way. But the behaviour has been implemented in other areas for a very long time - for instance in IM clients.
There is also the danger of mixing the two models, such as here on SE sites. ...

I find that putting it on the top is often distracting. on the other hand a site with a lot of comments forces a lot of scrolling if the form is on the bottom.
I like @Dan's idea of considering the audience but would add an idea.
If you do put it on the top make sure it is clean and simple so as not to distract from the comments below. If you put it at the ...

I work at a reviews software company, and we've found that it's a matter of authenticity. User profiles, avatars, number of votes, comments, etc, are all indicators that real users are contributing. If the brand says, "we like this review!" it's probably because it's positive. Users don't want to trust marketing copy or advertising, and if user generated ...

I've approached this problem, by not indenting, in some situations, and using arrows to indicate which comment replies to which comment.
Here's an example, from an open source discussion system I'm developing:
Here's a link to the above example, in real life: http://www.debiki.com/-71cs1#post-116979
(I also wrote a blog article about this:
...

It's a tradeoff. Chronological order is better for readability of threads, worse for diminishing the value of comments from latecomers, and for enabling early-bird campers to permanently stamp their agenda on the page.
This sums the first readability problem up neatly (credit: someone's forum signature, I forget where I saw it):
They break the ...

While I agree with the other answers that you shouldn't impose an arbitrary small limit since you want to give users the freedom to fully express themselves.
I will propose an alternate view related to security that there should be a hard upper limit to protect your systems. If you allow unlimited text to be entered it might be possible for a would be ...

One of the main advantages for requiring the user to enter in their email address when commenting is actually for the site administrator. There may be instances where your comment prompts a response not suited to, or too large for the comments section, and as such they will contact you directly.
It also acts as an extra barrier (although not a very good ...

You can use background color to represent going deeper in the hierarchy and avoid using too much indentation.
If you do not want to color the entire background of the post, you can do just highlighting the edge with a color.
In any case, I would still have the tiniest amount of indenting because it is a much stronger visual cue. You can compensate the ...